1. Field of the Invention
The present invention relates to laser shock processing, and more specifically, it relates to techniques for contouring metal by laser peening.
2. Description of Related Art
Using high power lasers to improve material properties is one of the most important industrial applications of lasers. Lasers can transmit controllable beams of high-energy radiation for metalworking. Primarily, the laser can generate a high power density that is localized and controllable over a small area. This allows for cost effective and efficient energy utilization, minimizes distortions in surrounding areas, and simplifies material handling. Since the laser pulse involves the application of high power in short time intervals, the process is adaptable to high-speed manufacturing. The fact that the beam can be controlled allows parts having complex shapes to be processed. Also accuracy, consistency, and repeatability are inherent to the system.
Improving the strength of metals by cold working undoubtedly was discovered early in civilization, as ancient man hammered out his weapons and tools. Since the 1950s, shot peening has been used as a means to improve the fatigue properties of metals. Another method of shock processing involves the use of high explosive materials in contact with the metal surface.
The use of high intensity laser outputs for the generation of mechanical shock waves to treat the surfaces of metals has been well known since the 1970s. The laser shock process can be used to generate compressive stresses in the metal surfaces, adding strength and resistance to corrosive failure.
Lasers with pulse outputs of 10 to 100 J and pulse durations of 10 to 100 ns are useful for generating inertially confined plasmas on the surfaces of metals. These plasmas create pressures in the range of 10,000 to 100,000 atmospheres and the resulting shock pressure can exceed the elastic limit of the metal and thus compressively stress a surface layer as deep or deeper than 1 mm in the metals. Lasers are now becoming available with average power outputs meaningful for use of the technique at a rate appropriate for industrial production.
In the process of laser shock processing, a metal surface to be treated is painted or otherwise made xe2x80x9cblackxe2x80x9d that is, highly absorbing of the laser light. The black layer both acts as an absorber of the laser energy and protects the surface of the part from laser ablation and from melting due to the high temperature of the plasma. A thin layer of water, typically 1 to 2 mm, is flowed over this black surface. The water acts to inertially confine or, as it is called, tamp the plasma generated as the laser energy is absorbed in the short time pulse duration, typically 30 ns. Other suitable materials that act as a tamper are also possible. A limitation to the usefulness of the process is the ability to deliver the laser energy to the metal surface in a spatially uniform beam. If not uniform, the highest intensity area of the light can cause a breakdown in the water which blocks delivery of meaningful energy to the painted metal surface. A conventional technique to deliver the laser light to the surface is to use a simple lens to condense the laser output to a power density of roughly 100 J to 200 J per square centimeter. This condensing technique has the limitation that a true xe2x80x9cimagexe2x80x9d of the laser near-field intensity profile is not obtained at the surface. Rather a field intensity representing something between the near and far fields is generated. Diffraction of the laser beam as it is focused down onto the surface results in very strong spatial modulation and hot spots.
Any phase aberrations generated within the beam, especially those associated with operation of the laser for high average power, can propagate to generate higher intensity areas within the beam. These high peak intensity regions cause breakdown in the water layer, preventing efficient delivery of the laser energy to the surface to be treated. Another potential cause of breakdown in the tamping material is the generation of non-linear effects such as optical breakdown and stimulated scattering. In a normal generation of a 10 ns to 100 ns pulse within a laser, the output slowly builds over a time period exceeding several pulsewidths. This slow, weak intensity helps to seed the non-linear processes that require buildup times of 10s of nanoseconds. In conventional techniques, the pulse output of the laser is xe2x80x9cslicedxe2x80x9d by an external means such as a fast rising electro-optical switch or by an exploding foil. These techniques can be expensive and can limit reliability.
A controlled application of compressive stress applied to one side of a metal surface will cause that surface to expand in a predictable manner and can thus curve the metal in a highly controllable fashion. Upon curving, the convex surface is left with a residual compressive stress, which is highly desirable for fatigue and corrosion resistance of the part in operation. The technique of inducing this compressive stress by means of shot peening is well known and in general use. However, shot peening is limited in the depth of intense compressive stress that can be induced without generating significant and undesirable cold working of the surface layer. Due to the required spherical shape of shot used for peening, the process imparts a non-uniform pressure versus time profile to the metal during each individual impact of the shot. Pressure is initiated at the first contact point of the sphere and then spreads across the impact area as the metals deform and the entire cross-section of the shot contacts the metal. This non-uniform application of pressure results in a local extrusion of the metal, a flow of metal from the center to the outer area of the impact zone. Consequently, more cold work is done on the metal as material extrudes due to the wedge of pressure created by the impact of the shot
The forming of metals into complex shapes is required for many applications. There are a number of processes that use heat and yielding to stretch and form metal into required shapes. Especially in the aerospace industry, the application of heat and yielding strain can be detrimental to the mechanical properties of the metal and hence undesirable for many metals such as aluminum and titanium. Nonetheless, the aerospace industry needs to form complex shaped parts and achieve the forming with a high degree of precision. Currently there are problems attaining the desired curvature without yielding in components of greater than half-inch thickness. There is also in general a lack of precise control in the parts forming such that attachment holes and the trimming to exact shape cannot be done until the formed part is brought to the component frame for final installation. Precise forming, with large curvature, especially in thick section materials, would be highly desirable.
The laser peening process as described in the parent application, can be used to shape components by creating residual compressive stress over a given surface area and allowing this stress to create strain resulting in a convex surface curvature. By systematically applying this process to both top and bottom surfaces of a piece of metal, the desired curvature for the entire piece of metal, including complex shapes such as saddle shapes can be achieved.
It would be desirable to provide a technique that achieves a greater curvature than that achieved with laser forming.
It is an object of the present invention to provide a means to achieve an enhanced curvature in a part by pre-loading the component in a local area with a bending moment that results in strain that deflects the part achieving a stress that is near to but below the yielding limit.
The parent application discussed the concept of forming complex shapes in metals by means of laser shot peening. That application teaches that deformation in any desired direction can be enhanced by mechanically inducing a bending moment (although below the yield limit of the metal) in that direction during application of the laser peenforming pulses. This continuation-in-part provides a more detailed description of this enhancement. The advantage of the applied bending moment can be very significant, adding up to 50% improvement with respect to the resulting net deformation and/or curvature. A simple means to induce a bending moment is to employ a fixture consisting of a strongback on the eventual concave side and oriented along the desired arc with grips to attach to the metal on the convex side. The strongback is drilled and threaded in one or more places to accept large bolts that can be screwed through the strongback and to push against a pad in contact with the metal to be formed. Computer controlled hydraulic actuators can be used in place of screws and be more effective in rapidly and exactly applying the desired loading. The pad is contoured to spread the load along the desired arc. Loading is achieved by turning the bolt with a large wrench until achieving a deformation equivalent to the desired percentage of a yielding deformation. With the component thus loaded, laser peening is applied to the convex side along the direction of the deformed arc. As the laser peening is applied to the surface, the induced stress begins to relax the resistance to the applied load. To obtain maximum deformation the bolt is adjusted to keep the loading constant. In the case of the hydraulic load, the computer control maintains a constant load. The laser peening pattern density and the laser intensity used depends on the material properties and desired residual curvature of the material being contoured. For complex shapes, the fixture and peening process are used along each desired arc of curvature.